1. Field of the Invention
The present invention relates to generation of electrical waveforms, and more specifically to a method and apparatus for generating a waveform having one signal level periodically and different signal levels in other durations.
2. Related Art
There is often a need to generate an electrical waveform having one signal level periodically and different signal levels in other durations. For example, in a technique known as correlated double sampling (CDS), the value of an information element (digital value or analog signal strength) is encoded as a difference of a fixed signal level transmitted during one duration (e.g., phase of a clock signal) and another signal level transferred during an adjacent duration.
Thus, when a sequence of information elements are to be represented, the corresponding waveform generally has a fixed signal level in alternative durations, and the signal level of the waveform in the remaining durations depends on the specific information element sought to be represented.
CDS is used in several technologies such as image capturing/processing (e.g., in a camera), in which each pixel of a charge coupled device (CCD) captures the light intensity of a corresponding point of an image in the form of charge, and the light intensity of successive points is transmitted in the form of a waveform represented using CDS. A CDS sampler may convert the waveform portions into successive voltage levels, which are then converted into digital samples by an analog to digital converter (ADC), as is well known in the relevant arts.
One example scenario in which such an electrical waveform may need to be generated is in testing a CDS sampler, and the desired waveform may be referred to as a CCD waveform. It may be desirable to test the CDS sampler for all possible voltage output levels (as represented by ramp output 104 in FIG. 1A). The corresponding input signal to the CDS sampler is shown as CCD waveform 103. In FIG. 1A, CDS waveform 103 is shown at fixed voltage 105 during alternate durations 101, and the desired voltage levels are shown represented in remaining durations 102. Thus, it may be desirable to generate a waveform such as CCD waveform 103.
In one approach of generating CCD waveform 103, a sequence of digital values are provided as an input to a digital-to-analog converter (DAC), with alternative digital values corresponding to fixed voltage 105. The remaining digital values are designed to respectively equal the voltage levels desired in durations 102. The DAC converts each received digital value to corresponding voltage level, and the output of the DAC represents CCD waveform 103.
One problem with such an approach is that a DAC may require a substantial amount of time (“settling time”) to settle at a final voltage level, particularly when the difference between successive durations is high (e.g., right after time point 106). The higher settling times are particularly problematic when a CDS sampler needs to operate at high speed (e.g., with a time period of less than 30 nano-seconds) and/or with high resolution (e.g., 12 bit resolution) because higher speed implies the length of durations 101 and 102 is short and higher resolution implies that the waveform is to have settled to a corresponding degree of closeness to the final level quickly.
At least for such reasons, an improved approach may be desirable to generate a waveform having one signal level periodically and different signal levels in other durations.